1 00:00:01 --> 00:00:05 Recall, please, that we were discussing last time 2 00:00:05 --> 00:00:10 the fact that the immune system makes a wide diversity of antibody 3 00:00:10 --> 00:00:15 molecules. And, by the way, a synonym for an 4 00:00:15 --> 00:00:20 antibody molecule is an immunoglobulin. 5 00:00:20 --> 00:00:25 Recall that we used that word very briefly. Another word we use, 6 00:00:25 --> 00:00:30 by the way, was the word antigen. And, an antigen is a functional term. 7 00:00:30 --> 00:00:34 That was pretty funny. An antigen is a functional term. 8 00:00:34 --> 00:00:39 An antigen is an agent that provokes an immune response. 9 00:00:39 --> 00:00:43 And, the term antigen doesn't really refer to any specific 10 00:00:43 --> 00:00:48 chemical structure. It doesn't even have to be a 11 00:00:48 --> 00:00:53 protein. Antigen simply refers to some chemical entity that creates 12 00:00:53 --> 00:00:57 some response from the immune system. We also use the word in 13 00:00:57 --> 00:01:02 passing, epitope. And, an epitope refers to a small 14 00:01:02 --> 00:01:06 region, for example, a protein against which an antibody 15 00:01:06 --> 00:01:11 has developed reactivity. And if you recall our discussion 16 00:01:11 --> 00:01:15 last time, a protein obviously of some size has multiple distinct 17 00:01:15 --> 00:01:19 epitopes, each of which can be recognized by a distinct antibody 18 00:01:19 --> 00:01:24 molecule. And, for us, could we, 19 00:01:24 --> 00:01:28 I'm sorry if I got all this political discussion 20 00:01:28 --> 00:01:32 started, thank you. Thank you. An epitope, 21 00:01:32 --> 00:01:36 from the point of view of a protein is an oligo peptide. 22 00:01:36 --> 00:01:40 So, a stretch of amino acids of maybe ten, or 12, 23 00:01:40 --> 00:01:44 or 15 amino acids forms an epitope. And therefore, you can imagine 24 00:01:44 --> 00:01:48 there are multiple epitopes on the surface of a protein. 25 00:01:48 --> 00:01:52 In fact, antibodies can sometimes recognize the internal parts of the 26 00:01:52 --> 00:01:56 protein because protein can sometimes become denatured 27 00:01:56 --> 00:02:00 or degraded. Indeed, as we will discuss shortly, 28 00:02:00 --> 00:02:04 proteins also become cleaved into oligo peptides. 29 00:02:04 --> 00:02:08 These oligo peptides can come from any part of the protein, 30 00:02:08 --> 00:02:12 to state the obvious, and therefore there might be internal epitopes 31 00:02:12 --> 00:02:17 that exist in a protein that are not normally exposed by the native 32 00:02:17 --> 00:02:21 protein. Recall that we were dealing with the issue of how the 33 00:02:21 --> 00:02:25 immune system is able to create a wide diversity of antibodies, 34 00:02:25 --> 00:02:30 immunoglobulins would share in common a constant domain. 35 00:02:30 --> 00:02:34 And the constant domain is formed by these invariant regions of the light 36 00:02:34 --> 00:02:38 and the heavy chains. We call this the heterotetramer in 37 00:02:38 --> 00:02:42 contrast to the variable domain up here which recognizes the antigen 38 00:02:42 --> 00:02:46 that initially provoked the production of this antibody molecule. 39 00:02:46 --> 00:02:50 And, the immune system may at any one point in time be making hundreds 40 00:02:50 --> 00:02:54 of thousands, maybe even millions or tens of millions of distinct 41 00:02:54 --> 00:02:58 antibody species that differ one from the other by the antigen 42 00:02:58 --> 00:03:03 recognition site here at this part of the antibody molecule. 43 00:03:03 --> 00:03:07 And recall as well that since these are proteins, the 44 00:03:07 --> 00:03:11 antigen-recognizing pocket is itself an oligopeptide, 45 00:03:11 --> 00:03:15 which combined in some complementary fashion to whatever antigen 46 00:03:15 --> 00:03:20 initially provoked the synthesis of this antibody molecule. 47 00:03:20 --> 00:03:24 At the end of last time, we discussed the fact that 48 00:03:24 --> 00:03:28 antibodies are made by B cells, and the fact that in the disease of 49 00:03:28 --> 00:03:33 multiple myeloma, one ends up with a monoclonal 50 00:03:33 --> 00:03:37 disease, that is to say, one of a vastly heterogeneous 51 00:03:37 --> 00:03:42 population of B cells begins to proliferate uncontrollably. 52 00:03:42 --> 00:03:46 And now, all of its descendants make a single antibody species. 53 00:03:46 --> 00:03:51 And that single antibody species consists once again of a heavy and a 54 00:03:51 --> 00:03:55 light chain, whose identity is created by the antigen-recognizing 55 00:03:55 --> 00:04:00 pocket that it happens to carry. And this begins to suggest a notion 56 00:04:00 --> 00:04:04 of clonal expansion. By that, I mean the following. 57 00:04:04 --> 00:04:08 Let's imagine a scenario where we start out with a naïve immune system 58 00:04:08 --> 00:04:12 where we have a whole series of different B cells. 59 00:04:12 --> 00:04:17 And this series goes here from 1-15, but in fact this could go from one 60 00:04:17 --> 00:04:21 up to a million. And each of these antibody 61 00:04:21 --> 00:04:25 producing B cells, or each of these B cells, 62 00:04:25 --> 00:04:30 has in principle the ability to respond to a different antigen. 63 00:04:30 --> 00:04:34 Let's just imagine that. And now, what we can imagine is 64 00:04:34 --> 00:04:38 that an antigen such as, for example, poliovirus comes into 65 00:04:38 --> 00:04:42 the body, and it is recognized by two different clones of these B 66 00:04:42 --> 00:04:47 cells. And that recognition, we can imagine, acts as a mytogenic 67 00:04:47 --> 00:04:51 signal, a growth stimulatory signal for these two particular clones and 68 00:04:51 --> 00:04:55 B cells. It almost acts as if it were a growth factor. 69 00:04:55 --> 00:05:00 But of course, we're not talking about growth factors here. 70 00:05:00 --> 00:05:04 We're talking about antigens, including antigens brought into the 71 00:05:04 --> 00:05:09 tissue by a foreign infectious agent. And therefore, 72 00:05:09 --> 00:05:13 the recognition of the antigen by these two B cells may thereafter 73 00:05:13 --> 00:05:18 provoke their clonal expansion. By that I mean to say that they 74 00:05:18 --> 00:05:23 will preferentially begin to proliferate whereas all the other B 75 00:05:23 --> 00:05:27 cells, which are not in any way responsive to that antigen will just 76 00:05:27 --> 00:05:32 sit there like bumps on a log. And, if there is, 77 00:05:32 --> 00:05:36 therefore, clonal expansion, now there will be a much larger 78 00:05:36 --> 00:05:40 number of cells in the immune system that are capable of producing the 79 00:05:40 --> 00:05:44 antibody that recognizes its provoking antigen. 80 00:05:44 --> 00:05:48 Again, recall that this number may go up to a million. 81 00:05:48 --> 00:05:52 This is just a great simplification. Now here, we use the word plasma 82 00:05:52 --> 00:05:56 cells. And in fact, if one wants to get the nomenclature 83 00:05:56 --> 00:06:00 very proper, the plasma cells are the products of B cells which mature 84 00:06:00 --> 00:06:04 into the antibody producing plasma cells. 85 00:06:04 --> 00:06:07 They are very close on the same lineage of cells, 86 00:06:07 --> 00:06:11 and what that means in the end for us is if once there's an active 87 00:06:11 --> 00:06:15 immune response, there's been the preferential 88 00:06:15 --> 00:06:18 expansion of certain sub-populations of B cells and plasma cells, 89 00:06:18 --> 00:06:22 and other ones of them which don't recognize these antigens just sit 90 00:06:22 --> 00:06:26 there are underrepresented in the total population of cells. 91 00:06:26 --> 00:06:30 Still, the fundamental question that we posed last time 92 00:06:30 --> 00:06:34 was the following. If it's the case, 93 00:06:34 --> 00:06:38 and it happens to be that each of these B cells makes an antibody 94 00:06:38 --> 00:06:42 having a distinct reactivity, a distinct ability to react to one 95 00:06:42 --> 00:06:46 or another antigen how do they make so many different kinds of antibody 96 00:06:46 --> 00:06:50 molecules. How do they know how to do that. We argued that it's 97 00:06:50 --> 00:06:54 implausible that the amino acid sequences of each antibody molecule 98 00:06:54 --> 00:06:58 are encoded in the germ line. Why? Well, if there are indeed a 99 00:06:58 --> 00:07:02 million or even 100 million distinct antibodies can be made by the immune 100 00:07:02 --> 00:07:06 system, clearly there can't be 100 million genes, 101 00:07:06 --> 00:07:10 each of which is dedicated to the production of a distinct 102 00:07:10 --> 00:07:14 antibody molecule. And the solution to this puzzle was 103 00:07:14 --> 00:07:18 first worked out by Susumu Tonegawa already more than 20 years ago who 104 00:07:18 --> 00:07:22 happens to be in our biology department. And what he discovered 105 00:07:22 --> 00:07:26 was the following, that the way that the antibody 106 00:07:26 --> 00:07:30 producing genes are organized is really quite extraordinary 107 00:07:30 --> 00:07:35 and unusual. Here's the heavy chain of the gene, 108 00:07:35 --> 00:07:39 and keep in mind that there's a heavy and a light chain gene. 109 00:07:39 --> 00:07:43 The heavy and the light chain genes are in different chromosomes. 110 00:07:43 --> 00:07:48 And therefore, the genes that are responsible for encoding these two 111 00:07:48 --> 00:07:52 proteins, the heavy and the light chain, they're on different 112 00:07:52 --> 00:07:56 chromosomes, and encode by different genes. OK, so what Tonegawa 113 00:07:56 --> 00:08:01 discovered was the following, that the variable region of an 114 00:08:01 --> 00:08:05 antibody molecule which is at its end terminus is encoded by small 115 00:08:05 --> 00:08:09 segments of DNA that are carried in one very large genetic locus, 116 00:08:09 --> 00:08:14 an immunoglobulin locus. And what happens in humans is the 117 00:08:14 --> 00:08:19 following. There are in humans rather than mice, 118 00:08:19 --> 00:08:24 this number should be 100 in humans, there are 100 distinct V segments 119 00:08:24 --> 00:08:29 that are encoded in this antibody locus. Each one is roughly the same 120 00:08:29 --> 00:08:34 size, and they're located in a large tandem array. 121 00:08:34 --> 00:08:37 Similarly, there are in humans 30 of these desegments. 122 00:08:37 --> 00:08:41 The numbers change here if one goes from mouse to human, 123 00:08:41 --> 00:08:45 but that's irrelevant. And, there are six of the J 124 00:08:45 --> 00:08:49 segments. And, much of the antigen-recognizing 125 00:08:49 --> 00:08:53 capacity, much of the variable region is encoded in the green and 126 00:08:53 --> 00:08:57 the blue region of the gene, and the encoding happens as follows. 127 00:08:57 --> 00:09:01 There is a combinatorial fashion the 128 00:09:01 --> 00:09:05 random fusion of V, D, and J segments. There are 129 00:09:05 --> 00:09:10 specific, highly specialized enzyme that will choose one specific V 130 00:09:10 --> 00:09:14 segment, one specific D segment, and one specific J segment, 131 00:09:14 --> 00:09:18 ostensibly on a stochastic basis, a totally random basis, and fuse 132 00:09:18 --> 00:09:23 them together. And if that happens, 133 00:09:23 --> 00:09:27 and the intervening DNA sequences are discarded, 134 00:09:27 --> 00:09:31 then right away we can see that through the combinatorial 135 00:09:31 --> 00:09:36 mathematics that results from this, there is the possibility for having 136 00:09:36 --> 00:09:40 100 times 30, which is 3, 00, times six, which is already 180, 137 00:09:40 --> 00:09:44 00 distinct combinations encoding the reading frame of this particular 138 00:09:44 --> 00:09:49 antibody molecule. Here, I've just said V23, 139 00:09:49 --> 00:09:53 I guess, D7, J2. But again, let's imagine, which is 140 00:09:53 --> 00:09:57 approximately the case, that each of these combinations can 141 00:09:57 --> 00:10:02 be fused with roughly equal probability. 142 00:10:02 --> 00:10:06 What happens subsequently, then, is that this variable region 143 00:10:06 --> 00:10:10 sequence, which has been formed by fusion, is then the transcript that 144 00:10:10 --> 00:10:14 comes from that is spliced to the constant region which is towards the 145 00:10:14 --> 00:10:18 C terminus. So here, when we talk about the V, 146 00:10:18 --> 00:10:22 D, and J segments, we're talking about the segments that are encoding 147 00:10:22 --> 00:10:26 this portion. Here's this portion of the antibody molecule. 148 00:10:26 --> 00:10:30 Keep in mind, from here on down, we're talking about constant region 149 00:10:30 --> 00:10:35 segments which are not affected by these combinatorial fusions. 150 00:10:35 --> 00:10:39 And, in the overhead I just showed you, I was just talking about how 151 00:10:39 --> 00:10:43 the heavy chain gene is rearranged. And what that results as a 152 00:10:43 --> 00:10:47 consequence is the following. This is just another version of 153 00:10:47 --> 00:10:52 what I just showed you before, where the V, D, and J segments 154 00:10:52 --> 00:10:56 become fused randomly to one another creating a V, D, 155 00:10:56 --> 00:11:00 J fusion thing, which then by splicing is fused to the constant 156 00:11:00 --> 00:11:05 region which here is called C sub-mu. 157 00:11:05 --> 00:11:08 For reasons of why it's called sub-mu I'll mention in a moment. 158 00:11:08 --> 00:11:12 And this is the actual messenger RNA which then goes into the 159 00:11:12 --> 00:11:16 cytoplasm. And what we can now imagine is the following, 160 00:11:16 --> 00:11:19 that when the immune system first begins, it randomly fuses whole 161 00:11:19 --> 00:11:23 series of V, D, and J segments together, 162 00:11:23 --> 00:11:27 and all kinds of combinations. The same happens in both the heavy 163 00:11:27 --> 00:11:31 chain and the light chain, although the light chain is slightly 164 00:11:31 --> 00:11:34 simpler. But that's irrelevant for us 165 00:11:34 --> 00:11:38 conceptually. And then, we end up having hundreds of 166 00:11:38 --> 00:11:42 millions of distinct V cells, each of which has a different 167 00:11:42 --> 00:11:46 combination of heavy and light chains because keep in mind the 168 00:11:46 --> 00:11:50 light chain genes are also being rearranged combinatorially. 169 00:11:50 --> 00:11:54 They're also being fused. And therefore, we can have 180, 170 00:11:54 --> 00:11:58 00 distinct heavy chains, and I forget the exact number for the 171 00:11:58 --> 00:12:02 number of light chains. It's a bit less, 172 00:12:02 --> 00:12:06 but we can multiply those two combinations by one another because 173 00:12:06 --> 00:12:10 keep in mind that the gene encoding the heavy chain here in its variable 174 00:12:10 --> 00:12:14 region, and the gene encoding the light chain in its variable region 175 00:12:14 --> 00:12:18 are organized on separate chromosomes. And, 176 00:12:18 --> 00:12:22 they're fused together. These segments are fused together 177 00:12:22 --> 00:12:26 randomly. So, the total number of antibody 178 00:12:26 --> 00:12:30 molecules we can make is 180, 00 distinct kinds of these. 179 00:12:30 --> 00:12:34 And, I've just slipped up in knowing the total number of these that can 180 00:12:34 --> 00:12:38 be made, although we could figure it out shortly if it were really 181 00:12:38 --> 00:12:42 important. But once again, the number is in the many thousands. 182 00:12:42 --> 00:12:46 And therefore, overall, the total number of distinct antibody 183 00:12:46 --> 00:12:50 molecules one can make on the basis of this is this. 184 00:12:50 --> 00:12:54 Did I get the math right, 60 by 30, 18,000. It's 18, 185 00:12:54 --> 00:12:58 00. So, there's 18,000 distinct of these that can be made, 186 00:12:58 --> 00:13:02 and the number of these, I think, is closer to 10,000. 187 00:13:02 --> 00:13:06 I forget, but multiplied, let's call it 10,000 for a moment. 188 00:13:06 --> 00:13:11 That's not the right number. There's 10,000 different ones of 189 00:13:11 --> 00:13:16 these, and 18, 00 of these that can be made. 190 00:13:16 --> 00:13:20 Multiply them together, and you get the total number of combinations of 191 00:13:20 --> 00:13:25 antibody molecules it could make because it is the case that the 192 00:13:25 --> 00:13:30 antigen recognition site here is created cooperatively, 193 00:13:30 --> 00:13:35 collaboratively, by the heavy and light chain. You had a question? 194 00:13:35 --> 00:13:39 So, the gene is rearranged, and when the rearrangement occurs, 195 00:13:39 --> 00:13:44 on V segment is fused to one D segment, is fused to one J segment. 196 00:13:44 --> 00:13:48 And all of the extraneous segments that are between them have been 197 00:13:48 --> 00:13:53 deleted from the chromosome. So, this leads me to a second point 198 00:13:53 --> 00:13:58 I wanted to make, and that is that here we are dealing 199 00:13:58 --> 00:14:03 with a process of somatic mutation. 200 00:14:03 --> 00:14:07 We talked about last time or the time before last the fact that 201 00:14:07 --> 00:14:12 somatic mutation is generally a process that can lead to cancer, 202 00:14:12 --> 00:14:17 i.e. the mutation of germ line genes that are in one way or another 203 00:14:17 --> 00:14:22 corrupted by random mutational processes. But here, 204 00:14:22 --> 00:14:27 I'm talking about a somatic mutational process, 205 00:14:27 --> 00:14:32 which is highly directed and highly organized, and focused on creating a 206 00:14:32 --> 00:14:37 series of rearranged genes that can then serve as the template for the 207 00:14:37 --> 00:14:42 production of an antibody molecule. So here, let's just look at this 208 00:14:42 --> 00:14:46 again. Here, I'll tell you, there are many V genes. There are 209 00:14:46 --> 00:14:50 many D genes, and there are many J genes. And, this is the embryonic 210 00:14:50 --> 00:14:54 DNA. After large segments of the immunoglobulin locus are deleted, 211 00:14:54 --> 00:14:58 and then the surviving V, D, and J segments become fused in the DNA 212 00:14:58 --> 00:15:02 directly to one another. And when that happens, 213 00:15:02 --> 00:15:06 the resulting B cell DNA can be transcribed, and via splicing create 214 00:15:06 --> 00:15:11 an mRNA where the region in between the J's chain and the constant 215 00:15:11 --> 00:15:16 region chain called, here C, is further deleted. 216 00:15:16 --> 00:15:20 So, each messenger RNA has one V, one D, and one J segment together 217 00:15:20 --> 00:15:25 with a constant region segment. There are yet other processes that 218 00:15:25 --> 00:15:30 further diversify how many different variable regions can be encoded. 219 00:15:30 --> 00:15:34 One of them is the following. It turns out that the process by 220 00:15:34 --> 00:15:38 which the V and D, and the D and J genes are fused 221 00:15:38 --> 00:15:42 together, those segments are fused together, is a bit sloppy. 222 00:15:42 --> 00:15:46 And therefore, sometimes the fusion will occur creating a sequence of 223 00:15:46 --> 00:15:50 nucleotides that has a nonsense codon in. Sometimes it will create 224 00:15:50 --> 00:15:54 a coherent reading frame that creates yet another kind of amino 225 00:15:54 --> 00:15:58 acid sequence there. And therefore, this is not a highly 226 00:15:58 --> 00:16:02 ordered process in the sense that these micro-architecture of the 227 00:16:02 --> 00:16:06 points with the V and the D, and the D and the J are fused is a 228 00:16:06 --> 00:16:10 little bit chaotic. So that creates even further 229 00:16:10 --> 00:16:16 randomization of the nucleotide sequence. And then, 230 00:16:16 --> 00:16:21 finally, once this gene has been created, there is a process which is 231 00:16:21 --> 00:16:27 called hypermutation, where through mechanisms which are 232 00:16:27 --> 00:16:32 not well understood, the nucleotide sequence of this 233 00:16:32 --> 00:16:38 region is actually further changed. 234 00:16:38 --> 00:16:44 It's mutated by certain enzymes. For example, there's an enzyme 235 00:16:44 --> 00:16:50 called cytidine deaminase, which takes off the amine group off 236 00:16:50 --> 00:16:56 of the cytidines in that region, and thereby effectively converts the 237 00:16:56 --> 00:17:02 C's in that coding region more into having a T coding capacity. 238 00:17:02 --> 00:17:06 This happens somatically, and here once again I'm talking 239 00:17:06 --> 00:17:10 about a further dimension of diversification. 240 00:17:10 --> 00:17:14 One of the dimensions diversification, 241 00:17:14 --> 00:17:18 first of all, the V and the J chains rearrange independently, 242 00:17:18 --> 00:17:22 randomly, and stochastically. Secondly, the joining of the 243 00:17:22 --> 00:17:26 segments is a bit sloppy. And thirdly, there is somatic 244 00:17:26 --> 00:17:30 hypermutation. There's an enzyme like this which 245 00:17:30 --> 00:17:34 is actually an actively mutagenic enzyme which fiddles around with 246 00:17:34 --> 00:17:38 some of the C's in this portion of the gene, thereby further 247 00:17:38 --> 00:17:42 diversifying the coding ability of the immunoglobulin gene that's 248 00:17:42 --> 00:17:46 encoded by the cell. And what this means is the following, 249 00:17:46 --> 00:17:51 that if we imagine this scheme here where each of these B cells 250 00:17:51 --> 00:17:56 recognize it's a different antigen, initially there may be 100 million 251 00:17:56 --> 00:18:01 distinct B cell clones that are created, each the consequence of a 252 00:18:01 --> 00:18:06 different random mutation. That happens early in the 253 00:18:06 --> 00:18:10 development of the immune system. Thereafter, there are certain 254 00:18:10 --> 00:18:14 selective processes there at play. One important selective process is 255 00:18:14 --> 00:18:18 the elimination of B cells that have failed to make nicely rearranged 256 00:18:18 --> 00:18:23 antibody molecules. What do I mean by that? 257 00:18:23 --> 00:18:27 I told you how sloppy the V, D, J joining is. And therefore, 258 00:18:27 --> 00:18:31 many of these joinings could create reading frames that are incoherent 259 00:18:31 --> 00:18:35 with stop codons in the middle. And therefore, 260 00:18:35 --> 00:18:39 those cells which happen to have created antibody molecules which are 261 00:18:39 --> 00:18:43 highly mutant and clearly structurally defective are 262 00:18:43 --> 00:18:46 eliminated. So, one prerequisite for the survival of 263 00:18:46 --> 00:18:50 these B cells early in the development of the immune system is 264 00:18:50 --> 00:18:54 that they've learned how to make functional heavy and light chains. 265 00:18:54 --> 00:18:58 If they don't, they're right away wiped out. They're eliminated. 266 00:18:58 --> 00:19:02 Here's another very important prerequisite for survival, 267 00:19:02 --> 00:19:06 that these B cells don't make antibodies that react with antigens 268 00:19:06 --> 00:19:11 that are native to the body's own tissues. What do I mean by that? 269 00:19:11 --> 00:19:15 Well, one of the wonders of the immune system is the following, 270 00:19:15 --> 00:19:20 that it can recognize foreign antigens that are brought into the 271 00:19:20 --> 00:19:24 body from the outside including, for example, the epitopes on the 272 00:19:24 --> 00:19:29 surface of polio virus particle. It can recognize many different 273 00:19:29 --> 00:19:33 viruses, surface antigens present on the surface of bacteria, 274 00:19:33 --> 00:19:38 and fungi, and all kinds of other infectious agents. 275 00:19:38 --> 00:19:43 But importantly, the immune system also sees hundreds 276 00:19:43 --> 00:19:48 of millions of different epitopes of the proteins that are present 277 00:19:48 --> 00:19:53 endogenously, native proteins in our tissues. A priori, 278 00:19:53 --> 00:19:58 the native proteins, what's called our own self-proteins, 279 00:19:58 --> 00:20:03 are perfectly qualified to function as antigens. 280 00:20:03 --> 00:20:07 And yet, the immune system rarely develops strong reactivity against 281 00:20:07 --> 00:20:12 them. Thus, the immune system has the ability to recognize self versus 282 00:20:12 --> 00:20:16 non-self. What do I mean by self versus non-self? 283 00:20:16 --> 00:20:21 Self are the body's own native proteins, which are in principal 284 00:20:21 --> 00:20:25 conceivably antigenic. Non-cell are foreign invaders that 285 00:20:25 --> 00:20:30 bring strange epitopes into the cell. 286 00:20:30 --> 00:20:35 And what happens is that B cells, which make antibodies, that 287 00:20:35 --> 00:20:41 recognize native proteins in our tissues are, if things are working 288 00:20:41 --> 00:20:46 well, rapidly eliminated early in the development of the immune system. 289 00:20:46 --> 00:20:52 What happens if that elimination fails to occur properly? 290 00:20:52 --> 00:20:58 If it fails to occur, one has the process of autoimmune disease. 291 00:20:58 --> 00:21:04 And, there are multiple autoimmune diseases. 292 00:21:04 --> 00:21:08 Type one diabetes, early onset diabetes happens when 293 00:21:08 --> 00:21:12 the immune system recognizes antigens that are present in the 294 00:21:12 --> 00:21:16 islet cells in the pancreas that make insulin. Rheumatoid arthritis, 295 00:21:16 --> 00:21:20 which inflicts a large portion of the elderly, happens when the immune 296 00:21:20 --> 00:21:24 system recognizes antigens that are present normally in the cartilage in 297 00:21:24 --> 00:21:28 our joints. Lupus happens when, it's an autoimmune called lupus 298 00:21:28 --> 00:21:32 which is often fatal, happens when the immune system 299 00:21:32 --> 00:21:37 recognizes proteins in many different tissues. 300 00:21:37 --> 00:21:41 And once again, in all of these cases, 301 00:21:41 --> 00:21:45 there is a breakdown of the mechanisms governing immune 302 00:21:45 --> 00:21:49 tolerance. And what do I mean by tolerance? I mean the mechanisms 303 00:21:49 --> 00:21:53 whereby the immune system tolerates native antigens, 304 00:21:53 --> 00:21:57 but is conversely intolerant of foreign antigens. 305 00:21:57 --> 00:22:01 The word intolerant is not used in immunology, but I'm just using it 306 00:22:01 --> 00:22:06 for the sake of explanation here. So, immune tolerance is a very 307 00:22:06 --> 00:22:10 important area of current immunological research. 308 00:22:10 --> 00:22:14 We don't really understand why we don't have much more autoimmune 309 00:22:14 --> 00:22:18 disease than we do. And by the way, only listed two or 310 00:22:18 --> 00:22:22 three of the most common kinds of autoimmune diseases that happen with 311 00:22:22 --> 00:22:26 the mechanisms that normally guarantee immune tolerance failed. 312 00:22:26 --> 00:22:30 And therefore, these B cells survive are ones that make 313 00:22:30 --> 00:22:34 functional antibodies and those whose antibodies do not recognize 314 00:22:34 --> 00:22:38 self-antigens. They are permitted to survive, 315 00:22:38 --> 00:22:43 and once again, we imagine that those that happened to make 316 00:22:43 --> 00:22:47 antibodies that recognize particular foreign antigens undergo or enjoy 317 00:22:47 --> 00:22:52 clonal expansion. During the development of the 318 00:22:52 --> 00:22:57 immune response, there is a further diversification 319 00:22:57 --> 00:23:02 of the antigen-recognizing domain of the protein by hypermutation. 320 00:23:02 --> 00:23:06 And therefore, descendants of these initially 321 00:23:06 --> 00:23:11 developed cells, which have begun to expand because 322 00:23:11 --> 00:23:16 they make an antibody, may develop antibody molecules that 323 00:23:16 --> 00:23:21 are even more able to bind avidly to the antigen. It could be that 324 00:23:21 --> 00:23:25 initially, these antibody-producing cells make antibody molecules that 325 00:23:25 --> 00:23:30 bind nicely to the antigen but not really avidly. Avidly means 326 00:23:30 --> 00:23:35 really tightly. And, that's enough to get their 327 00:23:35 --> 00:23:39 clonal expansion going, but during the process of clonal 328 00:23:39 --> 00:23:44 expansion, this hypermutation creates further variants of these 329 00:23:44 --> 00:23:48 cells, further mutates their antibody producing genes so that 330 00:23:48 --> 00:23:53 some of the descendants of these cells will produce antibody 331 00:23:53 --> 00:23:57 molecules that bind even more tightly and specifically to the 332 00:23:57 --> 00:24:01 provoking antigen. And when that happens, 333 00:24:01 --> 00:24:05 the quality of the antibodies is improved progressively. 334 00:24:05 --> 00:24:09 Now clearly, the somatic hypermutation once again, 335 00:24:09 --> 00:24:13 it's a random hypermutation. And those clones of cells in which 336 00:24:13 --> 00:24:16 the hypermutation created less effective antibodies will not have 337 00:24:16 --> 00:24:20 their proliferation stimulated. Those clones of cells whose 338 00:24:20 --> 00:24:24 antibodies make more and more tightly binding antibodies will 339 00:24:24 --> 00:24:28 preferentially have their proliferation stimulated. 340 00:24:28 --> 00:24:32 And as a consequence, they will now be the ones that are 341 00:24:32 --> 00:24:37 favored to yield the plasma cells that produce large amounts of 342 00:24:37 --> 00:24:42 antibody molecules. So, we have now this very unusual 343 00:24:42 --> 00:24:47 and very interesting way by which B cells and plasma cells are able to 344 00:24:47 --> 00:24:52 create a wide diversity of antibody molecules. Now, 345 00:24:52 --> 00:24:56 one of the interesting things, actually, is the fact that this 346 00:24:56 --> 00:25:01 constant region contains one of a variety of distinct 347 00:25:01 --> 00:25:06 constant regions. Initially, when one produces the 348 00:25:06 --> 00:25:10 first immune response, the first immune response as we said 349 00:25:10 --> 00:25:15 before produces a constant region which is called CM, 350 00:25:15 --> 00:25:20 and I had the right overhead this morning. Ah, here it is. 351 00:25:20 --> 00:25:24 Here's the way the genes are actually rearranged. 352 00:25:24 --> 00:25:29 Here's the rearranged V, D, and J, and here's the splice to 353 00:25:29 --> 00:25:34 the constant region. And downstream are a whole series of 354 00:25:34 --> 00:25:40 constant region segments. What's the first one: CM. 355 00:25:40 --> 00:25:45 I told you about that before. Thereafter, C sub delta, C sub 356 00:25:45 --> 00:25:51 gamma-3, gamma-1, gamma-2, gamma-8, and so forth. 357 00:25:51 --> 00:25:57 And therefore, the initial event can create an ig-mu, an IgM 358 00:25:57 --> 00:26:02 antibody molecule. The mu region creates an IgM. 359 00:26:02 --> 00:26:07 Later, as the immune response develops further, 360 00:26:07 --> 00:26:13 this mu segment may become deleted. And now, the splice may occur to a 361 00:26:13 --> 00:26:18 delta or a gamma change. And therefore, if the intervening 362 00:26:18 --> 00:26:23 constant region is changed, you might get what's called an IgG. 363 00:26:23 --> 00:26:28 An IgG is an immunoglobulin which is produced when this 364 00:26:28 --> 00:26:34 antigen-recognizing region becomes spliced down to here or here by 365 00:26:34 --> 00:26:39 virtue of the deletion of these intervening constant 366 00:26:39 --> 00:26:44 region segments. What's the purpose of what's called 367 00:26:44 --> 00:26:48 class switching? Because these are different classes 368 00:26:48 --> 00:26:52 of antibodies, because they have different 369 00:26:52 --> 00:26:56 functions. Note importantly that when this class switching occurs, 370 00:26:56 --> 00:27:00 it has no effect on the antigen-recognizing site of the 371 00:27:00 --> 00:27:04 antibody molecule. Rather, it affects the constant 372 00:27:04 --> 00:27:08 region of the antibody molecule. Well, how does that work? The 373 00:27:08 --> 00:27:13 initially made antibody molecule, I told you, is IgM. And in fact, if 374 00:27:13 --> 00:27:18 you look at the structure of IgM, it looks like this. Here's, let's 375 00:27:18 --> 00:27:22 say, a B cell, and the IgM molecule is actually not 376 00:27:22 --> 00:27:27 secreted into the extra cellular space like a soluble 377 00:27:27 --> 00:27:33 antibody molecule. Here's what IgM looks like from very 378 00:27:33 --> 00:27:39 schematically IgM has our standard antigen-recognizing site, 379 00:27:39 --> 00:27:45 but IgM is embedded in the plasma membrane of the B cell exactly the 380 00:27:45 --> 00:27:51 way that a growth factor receptor is embedded in the plasma membrane of a 381 00:27:51 --> 00:27:57 B cell. But keep in mind that I just said that this IgM molecule has 382 00:27:57 --> 00:28:03 an antigen-recognizing domain up here, which is exactly the way we 383 00:28:03 --> 00:28:10 described it before. So, the antigen recognition is not 384 00:28:10 --> 00:28:16 affected by whether we have an IgM molecule here. 385 00:28:16 --> 00:28:22 Only the location of the antibody molecule, and as I'll show shortly, 386 00:28:22 --> 00:28:28 the function of this antibody molecule is affected by this IgM. 387 00:28:28 --> 00:28:34 Later on, descendants of this B cell will mature into those that 388 00:28:34 --> 00:28:40 secrete IgG molecules into the plasma. 389 00:28:40 --> 00:28:44 So, here's a maturation that's going on. But look here. 390 00:28:44 --> 00:28:48 Once again, the antigen-recognizing domains of this IgG are unaffected 391 00:28:48 --> 00:28:53 by this conversion. Here, the constant region is 392 00:28:53 --> 00:28:57 tethering the antibody molecule to the plasma membrane. 393 00:28:57 --> 00:29:02 Here's the antigen-recognizing sites. 394 00:29:02 --> 00:29:06 After this maturation occurs, the secreted IgG molecules, which go 395 00:29:06 --> 00:29:10 into the solution, have identical antigen-recognizing 396 00:29:10 --> 00:29:14 sites, but now they're soluble proteins. Well, 397 00:29:14 --> 00:29:19 you'll say, why does the immune system do that? 398 00:29:19 --> 00:29:23 And, it goes back to an overhead I showed you just moments ago in which 399 00:29:23 --> 00:29:27 there was a puzzle implicitly created by the image on the overhead. 400 00:29:27 --> 00:29:32 And here it is. Let's look at this for a moment. 401 00:29:32 --> 00:29:37 What detail is now explained in this scheme? What detail is now 402 00:29:37 --> 00:29:43 explained? I told you that B cells can be stimulated by certain 403 00:29:43 --> 00:29:48 antigens. Let's imagine that each of these B cells made only secreted 404 00:29:48 --> 00:29:53 antibody, OK, instead of making cell surface antibody because the cell 405 00:29:53 --> 00:29:59 surface antibody, which is being made in this scheme, 406 00:29:59 --> 00:30:04 is actually the IgM molecules. Let's say this B cell over here is 407 00:30:04 --> 00:30:08 making an antigen. I'll just draw it as, 408 00:30:08 --> 00:30:12 there's the antigen, is making an antibody against this antigen, 409 00:30:12 --> 00:30:16 and it's secreting antibody against this. And I'd just argue that the B 410 00:30:16 --> 00:30:21 cell that makes good antibody is favored. It has its proliferation 411 00:30:21 --> 00:30:25 stimulated, right? That's what we talked about. 412 00:30:25 --> 00:30:29 But if this B cell sends all of its antibody into the plasma, 413 00:30:29 --> 00:30:34 how is it going to know that it's making a good antibody? 414 00:30:34 --> 00:30:38 It can't because all of the antigen recognition is happening by the 415 00:30:38 --> 00:30:43 secreted antibody. So, what happens with the IgM 416 00:30:43 --> 00:30:48 molecules. The first antibody molecule this produced is IgM. 417 00:30:48 --> 00:30:52 And, it stays tethered to the plasma membrane of the B cell. 418 00:30:52 --> 00:30:57 It's a transmembrane protein, and it functions much like a 419 00:30:57 --> 00:31:01 growth factor receptor. That is to say, 420 00:31:01 --> 00:31:05 when the antigen binds out here, there are signals that are radiated 421 00:31:05 --> 00:31:09 into the cell that induce the proliferation of this cell. 422 00:31:09 --> 00:31:13 And now, these cells that produce the antigen-recognizing IgM 423 00:31:13 --> 00:31:16 molecules on their surface can now have their proliferation stimulated. 424 00:31:16 --> 00:31:20 That solves the puzzle here which is created by this scheme. 425 00:31:20 --> 00:31:24 If all these B cells just made secreted antibody like this, 426 00:31:24 --> 00:31:28 then there would be no way to encourage their proliferation 427 00:31:28 --> 00:31:32 because there would be no way of telling this B cell you're 428 00:31:32 --> 00:31:36 doing a good thing. Keep making more of these antibody 429 00:31:36 --> 00:31:40 molecules. Here, the antigen-recognizing capability 430 00:31:40 --> 00:31:44 is embedded in the plasma membrane. And when an antigen binds this, the 431 00:31:44 --> 00:31:49 IgM molecule's organized so that now growth stimulatory signals are sent 432 00:31:49 --> 00:31:53 into the cell which cause this cell to begin to proliferate, 433 00:31:53 --> 00:31:57 producing more IgM containing cells which are further stimulated 434 00:31:57 --> 00:32:01 to proliferate. And ultimately after this has gone 435 00:32:01 --> 00:32:05 on for a while, there is a class switching which 436 00:32:05 --> 00:32:09 deletes the IgM region of the gene chain region, and causes a 437 00:32:09 --> 00:32:13 conversion to IgG. Again, the conversion from here to 438 00:32:13 --> 00:32:17 here doesn't change the antigen-recognizing capability of 439 00:32:17 --> 00:32:21 these two antibodies. It just makes a cell surface 440 00:32:21 --> 00:32:25 transmembrane protein or a secreted protein. (pause) 441 00:32:25 --> 00:32:37 No, the VDJ choice happens in the 442 00:32:37 --> 00:32:41 nucleus. It's a fusion of different DNA segments, and when it's 443 00:32:41 --> 00:32:46 transcribed, that fused VDJ is now spliced to a constant region segment. 444 00:32:46 --> 00:32:50 The choice of VDJ is a fusion of DNA segments. It happens in 445 00:32:50 --> 00:32:55 chromosomal DNA. It results in the deletion of all 446 00:32:55 --> 00:32:59 the segments that aren't used. So, all that's left in the 447 00:32:59 --> 00:33:04 immunoglobulin locus of a B cell is a DNA segment encoding V fused to a 448 00:33:04 --> 00:33:09 DNA segment encoding D, fused to a DNA segment encoding J. 449 00:33:09 --> 00:33:13 So now, you have a new somatically mutated immunoglobulin gene, 450 00:33:13 --> 00:33:18 lest there be any residual confusion about that. So, 451 00:33:18 --> 00:33:22 you see the elegance of this class-switching thing. 452 00:33:22 --> 00:33:27 Now there's yet other constant region segments that are used for 453 00:33:27 --> 00:33:31 other purposes. For example, when you have 454 00:33:31 --> 00:33:36 allergies, there's an activation of the igE antibodies. 455 00:33:36 --> 00:33:41 When you have secreted antibody production in the colon, 456 00:33:41 --> 00:33:46 igA is used more for that. So, different ones of these 457 00:33:46 --> 00:33:51 constant regions are used for different immunological applications. 458 00:33:51 --> 00:33:57 The IgM molecule, as I've said, is used here just as a 459 00:33:57 --> 00:34:02 way of telling the B cell that it's done well by making the right kind 460 00:34:02 --> 00:34:07 of antigen-recognizing site. The real business of creating 461 00:34:07 --> 00:34:11 antibodies is the soluble antibody production, the immunoglobulins, 462 00:34:11 --> 00:34:15 or the gammaglobulins because the vast majority of antibodies floating 463 00:34:15 --> 00:34:20 around the blood plasma are in fact IgGs. The IgMs are, 464 00:34:20 --> 00:34:24 as implied by this, actually just largely tethered to the surface of 465 00:34:24 --> 00:34:29 cells. So, this really is an extraordinary, 466 00:34:29 --> 00:34:33 elegant way of creating essentially hundreds of millions of different 467 00:34:33 --> 00:34:37 antigen-recognizing domains each created collaboratively between a 468 00:34:37 --> 00:34:42 heavy and a light chain. And once these antigen-recognizing 469 00:34:42 --> 00:34:46 domains are created through changes in DNA structure, 470 00:34:46 --> 00:34:50 then they can be used for different immunological applications, 471 00:34:50 --> 00:34:55 driving B cell proliferation, secreting soluble antibodies that 472 00:34:55 --> 00:34:59 are used to neutralize virus particles in the blood, 473 00:34:59 --> 00:35:03 or if they're hyperactive to create allergic reactions or to create 474 00:35:03 --> 00:35:08 immunity in certain regions of the gut and so forth. 475 00:35:08 --> 00:35:12 So, this class switching doesn't change the antigen-recognizing 476 00:35:12 --> 00:35:17 capability. It just changes the utilization of how already-developed 477 00:35:17 --> 00:35:21 VDJ segments and their antigen-recognizing capability are 478 00:35:21 --> 00:35:26 exploited by cells of the immune system. 479 00:35:26 --> 00:35:30 Now, one thing that's not really clear by all this is how all this 480 00:35:30 --> 00:35:35 develops at the cellular level because what I've talked about until 481 00:35:35 --> 00:35:40 now is called humeral immunity. Well, we talk about somebody having 482 00:35:40 --> 00:35:44 a good sense of humor. But the actual original meaning of 483 00:35:44 --> 00:35:49 the word humor in Latin was fluids that were fluxing through your body, 484 00:35:49 --> 00:35:54 and were responsible for your different mood states. 485 00:35:54 --> 00:35:58 So, if you were depressed or there just was a national election, 486 00:35:58 --> 00:36:03 there would be black fluids, black humors coursing through 487 00:36:03 --> 00:36:08 your blood. And if you were in a good mood, 488 00:36:08 --> 00:36:13 there were other humors coursing through your blood. 489 00:36:13 --> 00:36:19 And that leads through this etymology to the term of humeral 490 00:36:19 --> 00:36:24 immunity, which is to say the soluble immunity, 491 00:36:24 --> 00:36:29 i.e. the production of soluble antibody molecules. 492 00:36:29 --> 00:36:34 But I will tell you that there's a second arm of the immune system 493 00:36:34 --> 00:36:40 which is equally important, and that's called cellular immunity. 494 00:36:40 --> 00:36:44 And, to make a long story short, and we'll elaborate it on Monday 495 00:36:44 --> 00:36:48 briefly. Cellular immunity is largely created when you have a kind 496 00:36:48 --> 00:36:52 of immune cell that's called a cell rather than the B cells we've been 497 00:36:52 --> 00:36:56 talking about until now, which is able, and this cellular 498 00:36:56 --> 00:37:01 immunity is among other things, cytotoxic. 499 00:37:01 --> 00:37:07 And, it can recognize a second cell over here. Here's a second cell. 500 00:37:07 --> 00:37:13 This is its target cell because the second target cell is displaying on 501 00:37:13 --> 00:37:20 its surface certain antigens. So, antigens are being displayed on 502 00:37:20 --> 00:37:26 the surface of the target cell. I've indicated them here as just 503 00:37:26 --> 00:37:32 little, blue strokes. And the T cell is able to recognize 504 00:37:32 --> 00:37:36 these antigens on the surface of this cell here, 505 00:37:36 --> 00:37:41 and is able to kill this cell through a series of interesting 506 00:37:41 --> 00:37:45 mechanisms. This doesn't involve the intervention of soluble antibody 507 00:37:45 --> 00:37:50 molecules. Here, we're talking about one cell 508 00:37:50 --> 00:37:55 recognizing another, and it turns out to be very 509 00:37:55 --> 00:37:59 important for antiviral defenses. I've told you one way by which 510 00:37:59 --> 00:38:03 antiviral defenses are achieved. Soluble antibody molecules are 511 00:38:03 --> 00:38:07 secreted into the blood plasma like IgGs. They recognize an epitope on 512 00:38:07 --> 00:38:11 the surface of a virus particle. They glom onto that epitope on the 513 00:38:11 --> 00:38:15 surface of the virus particle, and they neutralize the virus 514 00:38:15 --> 00:38:19 particle. And, that's very important. 515 00:38:19 --> 00:38:23 We started out in our discussion of poliovirus talking about that. 516 00:38:23 --> 00:38:27 But this also turns out to be very important. Why is it important? 517 00:38:27 --> 00:38:31 For the following reason: when this cell, let's say this cell is 518 00:38:31 --> 00:38:35 infected by poliovirus on the inside. 519 00:38:35 --> 00:38:39 It turns out, interestingly enough, that the cell has a way which we'll 520 00:38:39 --> 00:38:44 discuss of processing poliovirus polypeptides, viral polypeptides, 521 00:38:44 --> 00:38:48 and displaying them on the cell surface. In other words, 522 00:38:48 --> 00:38:53 the cell can chew up some poliovirus proteins, put them to the outside, 523 00:38:53 --> 00:38:57 and tell the outside world, these are the kinds of the proteins that 524 00:38:57 --> 00:39:02 are being made right now inside of me. 525 00:39:02 --> 00:39:06 You the immune system can't really look through the plasma membrane so 526 00:39:06 --> 00:39:10 I'm going to tell you this is what's going on inside of me. 527 00:39:10 --> 00:39:14 There are poliovirus proteins being made as we speak. 528 00:39:14 --> 00:39:18 On the outside of the cell, these are displayed even though 529 00:39:18 --> 00:39:22 poliovirus replication is recurring exclusively inside the cell. 530 00:39:22 --> 00:39:26 A cytotoxic T cell here may recognize these antigens on the 531 00:39:26 --> 00:39:30 surface of the poliovirus infected cell, and proceed to kill the cell. 532 00:39:30 --> 00:39:34 Why is that interesting or important? Because the cytotoxic cell will 533 00:39:34 --> 00:39:39 kill the virus-infected cell while the virus infection is still in full 534 00:39:39 --> 00:39:43 swing. And therefore, by killing the virus-infected cell, 535 00:39:43 --> 00:39:48 it will abort the entire bioreplication cycle because the 536 00:39:48 --> 00:39:53 virus won't have enough time to replicate or proliferate inside the 537 00:39:53 --> 00:39:57 infected cell before it happens. It's preemptively killed by the 538 00:39:57 --> 00:40:02 cytotoxic T cell. And in fact, this way of eliminating 539 00:40:02 --> 00:40:08 viral infections from our body is as important and often more important 540 00:40:08 --> 00:40:13 than the neutralization of soluble virus particles in our plasma. 541 00:40:13 --> 00:40:18 And all that gets now to have us focus increasingly on the mechanisms 542 00:40:18 --> 00:40:24 by which antigens are recognized and processed so that the immune system 543 00:40:24 --> 00:40:29 can begin to respond to them. So, I want to get into that now, 544 00:40:29 --> 00:40:35 into the aspects in which we look at the cellular arms of 545 00:40:35 --> 00:40:41 the immune system. The most important thing initially 546 00:40:41 --> 00:40:47 in the immune response is that let's say poliovirus particle is 547 00:40:47 --> 00:40:54 recognized and digested by certain phagocytic cells of the immune 548 00:40:54 --> 00:41:00 system. What's a phagocytic cell: a cell that is able to gobble 549 00:41:00 --> 00:41:06 up other things. So, a well-known phagocytic cell is 550 00:41:06 --> 00:41:10 a macrophage. And there you can see the term phagocyte, 551 00:41:10 --> 00:41:14 macrophage. Phagos means to chew up or to swallow, 552 00:41:14 --> 00:41:19 so macrophage might see a particle like this surround that particle, 553 00:41:19 --> 00:41:23 and then internalize that particle and digest it, 554 00:41:23 --> 00:41:27 let's say a poliovirus particle. Macrophage could envelop a 555 00:41:27 --> 00:41:32 poliovirus particle and internalize it. 556 00:41:32 --> 00:41:36 It could do the same thing with a bacteria. Here, 557 00:41:36 --> 00:41:40 we'll use as an abbreviation for macrophage this. 558 00:41:40 --> 00:41:44 So, that's macrophage. There are yet other even more 559 00:41:44 --> 00:41:48 important phagocytic cells of the immune system that are called 560 00:41:48 --> 00:41:52 dendritic cells. Now, these cells gobble up whatever 561 00:41:52 --> 00:41:57 happens to be around them. They don't care. 562 00:41:57 --> 00:42:02 They are promiscuous sewer, gutter trollers. Whatever happens 563 00:42:02 --> 00:42:08 to be around, they will gobble up whatever happens to be around. 564 00:42:08 --> 00:42:14 They'll put it inside of them, and now they do something really 565 00:42:14 --> 00:42:19 interesting. The dendritic cells, the macrophages, the phagocytic 566 00:42:19 --> 00:42:25 cells, they don't digest the internalized particles down to amino 567 00:42:25 --> 00:42:31 acids. They digest the virus particle down to oligopeptides. 568 00:42:31 --> 00:42:37 Usually if 10, 12, 14 amino acids long: very important. 569 00:42:37 --> 00:42:43 So they don't make amino acids. They make oligopeptides. Why are 570 00:42:43 --> 00:42:49 they doing this digestion? Because these cells are 571 00:42:49 --> 00:42:55 exhibitionists and they want to show the rest of the world what they've 572 00:42:55 --> 00:43:02 just swallowed up. It's really important to them. 573 00:43:02 --> 00:43:07 Go figure. So, what do they do? 574 00:43:07 --> 00:43:12 They take these oligopeptides that they've just internalized and they 575 00:43:12 --> 00:43:17 put them on the cell surface via molecules which are called MHC class 576 00:43:17 --> 00:43:23 2. MHC class 2 is the name of these molecules, and these are cell 577 00:43:23 --> 00:43:29 surface receptors. And, these MHT C class 2 molecules 578 00:43:29 --> 00:43:35 have included in them an oligopeptide that has just been 579 00:43:35 --> 00:43:41 produced by the proteolytic cleavage of an internalized infectious agent, 580 00:43:41 --> 00:43:47 let's say. And so, the macrophage has many of these MHC class 2 581 00:43:47 --> 00:43:53 molecules on its surface, and it displays to the outside world 582 00:43:53 --> 00:44:00 what it has just captured in terms of these oligopeptides. 583 00:44:00 --> 00:44:05 And these oligopeptides are, by the way, roughly the size of an 584 00:44:05 --> 00:44:10 epitope. Therefore, this macrophage is just so excited. 585 00:44:10 --> 00:44:16 It's like a little kid who has just found something, 586 00:44:16 --> 00:44:21 so excited to show the world what it's just found and gobbled up and 587 00:44:21 --> 00:44:26 processed. And over here is a class of cells which are called T helper 588 00:44:26 --> 00:44:32 cells. And the T helper cells are interesting in their 589 00:44:32 --> 00:44:38 own right, right? The T helper cells have evolved 590 00:44:38 --> 00:44:45 through a mechanism that is similar to and parallel to the evolution of 591 00:44:45 --> 00:44:52 the B cells. As a consequence, there are many different kinds of T 592 00:44:52 --> 00:44:59 helper cells. Each one, so TH-1, TH-2, TH-3, each of which 593 00:44:59 --> 00:45:07 displays on its surface a T cell receptor. 594 00:45:07 --> 00:45:12 What kind of T cell receptor does it display? A T cell receptor that has 595 00:45:12 --> 00:45:18 gone through exactly the same kind of hoops that the antibody genes 596 00:45:18 --> 00:45:24 have gone through. That is, each T cell expresses on 597 00:45:24 --> 00:45:30 its surface an antigen-recognizing receptor just like the IgM molecule 598 00:45:30 --> 00:45:36 except it's made by T cells and it's created by these stochastic 599 00:45:36 --> 00:45:41 processes of fusing DNA segments. So, each of these T cells, 600 00:45:41 --> 00:45:45 and again, there are millions of them, has a different T cell 601 00:45:45 --> 00:45:49 receptor with a different antigen-recognizing capability. 602 00:45:49 --> 00:45:54 And here's what happens in the immune system. 603 00:45:54 --> 00:45:58 Let's imagine, as I say every year, 604 00:45:58 --> 00:46:03 that we're in a Middle Eastern bazaar, and that Middle Eastern 605 00:46:03 --> 00:46:07 bazaar, here are all the shops on the sides on the sides of this long 606 00:46:07 --> 00:46:12 bazaar. It's sometimes called a shuk or a suk. 607 00:46:12 --> 00:46:16 And here we're going to have a macrophage or a dendritic cell 608 00:46:16 --> 00:46:20 moving through the bazaar. Hanging out in front of the stores 609 00:46:20 --> 00:46:24 are a whole bunch of T helper cells. 610 00:46:24 --> 00:46:36 Business is slow, 611 00:46:36 --> 00:46:40 so all these T helper cells are just hanging out in front, 612 00:46:40 --> 00:46:44 and as is not the custom in this region, let's pretend all the T 613 00:46:44 --> 00:46:48 helper cells are girls just for the heck of it. It doesn't make any 614 00:46:48 --> 00:46:52 difference. And each of these T helper cells is displaying its own T 615 00:46:52 --> 00:46:56 cell receptor which has arisen through stochastic mechanisms and 616 00:46:56 --> 00:47:00 which recognizes a different oligopeptide. 617 00:47:00 --> 00:47:04 And here we have now the macrophage coming through like a vendor, 618 00:47:04 --> 00:47:09 a street vendor, saying, look girls, look at the epitope I just gobbled 619 00:47:09 --> 00:47:13 up and produced. Isn't it great? And, 620 00:47:13 --> 00:47:18 it's using its MHC class 2 molecules to present, it's like hands, 621 00:47:18 --> 00:47:22 presenting the epitopes. It's flogging them like a street vendor, 622 00:47:22 --> 00:47:27 and it walks down through this bazaar, and each of the T helper 623 00:47:27 --> 00:47:31 cells, they're congregating along the sides of the road, 624 00:47:31 --> 00:47:36 and most of them say look at this drip. 625 00:47:36 --> 00:47:40 Look at the garbage he's selling us today. I am totally uninterested. 626 00:47:40 --> 00:47:44 He was here last week with some other chump too. 627 00:47:44 --> 00:47:48 Let's hope he disappears quickly. And meanwhile the macrophage or the 628 00:47:48 --> 00:47:52 dendritic cell is very anxiously trying to find someone who has even 629 00:47:52 --> 00:47:56 the slightest bit of affection or respect for him. 630 00:47:56 --> 00:48:00 And this goes on for a long time. 631 00:48:00 --> 00:48:06 And finally, the macrophage finds a T helper cell over here. 632 00:48:06 --> 00:48:13 So, here's the T helper cell, and it turns out that by coincidence 633 00:48:13 --> 00:48:19 the T helper cell has on here surface a T cell receptor that 634 00:48:19 --> 00:48:26 recognizes the epitope that the macrophage is peddling. 635 00:48:26 --> 00:48:34 So, here's the macrophage. Macrophage has on its surface this 636 00:48:34 --> 00:48:42 oligopeptide held by the MHC class 2 hands. So, here's the peptide. 637 00:48:42 --> 00:48:51 And this T cell receptor which I'll abbreviate TCR recognizes, 638 00:48:51 --> 00:49:00 binds strongly to the epitope that's being flogged by the macrophage. 639 00:49:00 --> 00:49:04 And this is, I will tell you honestly, love at first sight. 640 00:49:04 --> 00:49:08 She says, oh, I can't believe it. You're selling an epitope that I 641 00:49:08 --> 00:49:12 just happen to love. This is very exciting. 642 00:49:12 --> 00:49:17 And all the other T helper cells say, oh my God, 643 00:49:17 --> 00:49:21 what does she see in him? And she gets all excited because 644 00:49:21 --> 00:49:25 there's a direct complementarity. In fact, to tell the truth, the T 645 00:49:25 --> 00:49:29 cell receptor recognizes not only the epitope but also the surrounding 646 00:49:29 --> 00:49:34 amino acids in the fingers of the MHC class 2 molecule. 647 00:49:34 --> 00:49:38 It's this constellation of things that's recognized by the T cell 648 00:49:38 --> 00:49:43 receptor and gets this T cell receptor really excited. 649 00:49:43 --> 00:49:47 And what does she do? Well, we can't talk about everything 650 00:49:47 --> 00:49:52 because this is polite company. But most important for what she 651 00:49:52 --> 00:49:56 does is she begins to proliferate like mad. Well, 652 00:49:56 --> 00:50:01 you know, cells have a limited repertoire of behavioral routines. 653 00:50:01 --> 00:50:05 Next time we're going to figure out how this leads to an immune response. 654 00:50:05 --> 50:10 So, we're going to be on [intention? all weekend.